© 2012 delmar, cengage learning chapter 6 advanced shielded metal arc welding

© 2012 Delmar, Cengage Learning

Chapter 6

Advanced Shielded Metal Arc Welding


Objectives

• Discuss how metal must be prepared before welding

• Describe the process, and demonstrate making the root pass, filler weld, and cover pass in all positions and techniques

• Explain the purpose of a hot pass• Tell what should be checked with a visual

inspection, and describe the appearance of an acceptable weld


Objectives (cont'd.)

• Demonstrate how to make: – A root pass on plate in all positions

– A root pass on plate with an open root in all positions

– An open root weld on plate using the step technique in all positions

– A multiple pass filler weld on a V-joint in all positions using E7018 electrodes

– A cover bead in all positions


Objectives (cont'd.)

– A single V-groove open root butt joint with an increasing root opening

– A single V-groove open root butt joint with a decreasing root opening

– SMAW welds of plate to plate

– SMAW welds of pipe to pipe


Introduction

• SMAW process produces high-quality welds– Welders frequently make welds in difficult situations

to a code or standard

• Metal edges must be prepared – 100% joint penetration

– Code-quality welds

– Metal thicker than ¼ inch

• Preparation improves strength – Prepared joints require more than one weld pass


Introduction (cont'd.)

• Root pass – Fuses and seals parts together

• Hot pass – Improves weld contour

• A test for one company may not qualify a welder for another company– AWS Certified Welder program

• Available from AWS's office in Miami, Florida


Root Pass

• First of a multiple pass weld– Fuses two parts together

– Establishes depth of weld metal penetration

– Needed to obtain a sound weld

– May be open or closed

– Can use a backing strip or backing ring• Backing strip used in a closed root can remain as

part of the weld or be removed• Removable backup tapes have been developed• Tape can be peeled off after weld is completed


FIGURE 6-2 Root pass maximum deposit 1/4 in. (6 mm) thick. © Cengage Learning 2012


Root Pass (cont'd.)

• Widely used in plate and pipe designs– Face side is not as important as root surface on

back or inside

– If root surface is correct: front side can be ground, gouged, or burned out

• Weld is evaluated from root side only

– Root face for most grooves will be about the same size


FIGURE 6-4 Using back gouging to ensure a sound weld root. © Cengage Learning 2012


Root Pass (cont'd.)

• Control penetration on joints with varying root gaps – Stepping electrode manipulation

– Key hole• Electrode is moved in and out of molten weld pool• Metal flows through key hole to root surface• Key hole ensures 100% penetration• Process requires more welder skill


FIGURE 6-7 Electrode movement to open and use a key hole. © Cengage Learning 2012


Hot Pass

• Surface of a root pass may be irregular• High-strength code welds

– Root pass and each filler pass must be ground• Grinding is important when high-strength, low alloy

welding electrodes are used

• Hot pass – Cleans out trapped slag

– Makes a root pass more uniform

– Uses high amperage and a fast travel rate

– Rapidly melts a large surface area


Hot Pass (cont'd.)

• Small amount of metal should be deposited– Resulting weld is concave

• Concave weld is easier cleaned• Failure to clean a convex root leaves wagon tracks

• Can be used to repair or fill small spots– Incomplete fusion or pinholes

• Normal weave pattern – Straight step or “T” pattern


FIGURE 6-19 Slag trapped between passes will show on an X ray. © Cengage Learning 2012


Hot Pass (cont'd.)

• Key points– Do not allow molten weld pool to cool completely

– Do not blow shielding gas covering away from the molten weld pool

– Penetration of the molten weld pool must be deep• Free all trapped slag• All porosity must be burned out


Filler Pass

• Fills groove after root pass– Made with stringer or weave beads

• Characteristics– Weld beads must overlap

• Stringer beads overlap 50%• Weave beads overlap 25%

– Finish bead is smooth

– Each weld bead must be cleaned before the next bead is started


FIGURE 6-24 Filler pass buildup sequence.© Cengage Learning 2012


Filler Pass (cont'd.)

• Ways to remove slag between filler weld passes– Chipping, wire brushing, and grinding

• Weld can be checked by ultrasonic or radiographic nondestructive testing– Most schools are not equipped to do this testing

– Check soundness by destructive testing


Cover Pass

• Last weld bead on a multipass – May use a different electrode weave

– Must be uniform and neat looking

– Appearance might be the only factor in accepting or rejecting welds

– Should not be more than 1/8 inch wider than the groove opening


FIGURE 6-26 The cover pass should not be excessively large. © Cengage Learning 2012


Plate Preparation

• Weld groove prepares the plate– Can be cut into one side or both sides

– May be cut into one or both plates of the joint

• Depth, angle, and location– Determined by a code standard

• SMA welds on plate 1/4 inch or thicker that need to have a weld with 100% joint penetration – Plate must be grooved

• May be ground, flame cut, gouged, or machined


Plate Preparation (cont'd.)

• Bevels and V-grooves – Best if cut before parts are assembled

• J-grooves and U-grooves – Can be cut either before or after assembly

• Groove on both sides– Tee joints, welds with little distortion, and welds that

will be loaded equally from both sides

• Back gouging – Cuts a groove in back side of a joint that has been

welded


Preparing Specimens for Testing

• Maximum allowable size for fissures in a guided-bend test – Given in codes for specific applications

• Some standards are listed– ASTM E190

– AWS QC10

– AWS QC11

• Copies available from appropriate organization


Acceptance Criteria for Face Bends and Root Bends

• Key points– Weld is uniform

– No arc strikes on the plate other than the weld

– Free of incomplete fusion and cracks

– Penetration must be 100% or as specified

– Weld must be free of overlap

– Correct weld specimen preparation is essential


Acceptance Criteria for Face Bends and Root Bends (cont'd.)

• Specimen cut out of test weldment– Abrasive disc

– Sawing

– Cutting with a torch

• Flame-cut specimens – Grind or smooth the edges

• All corners must be rounded– Radius of 1/8 inch maximum

– Grinding or machining marks must run lengthwise


Restarting a Weld Bead

• Welding bead must be restarted – After stopping to change electrodes

• Weld bead near completion– Should be tapered

• Increase the travel rate

• Before restarting– Chip slag and clean weld crater

• Restart the arc in the joint ahead of the weld– Electrodes must be allowed to heat up


Restarting a Weld Bead (cont'd.)

• Movement to root of weld and back up on bead– Builds up weld

– Reheats metal

• Avoid starting and stopping weld beads in corners– Tapering and restarting are especially difficult in

corners• Often results in defects


Preheating and Postheating

• Preheating – Application of heat to metal before welding

– Helps to reduce:• Cracking• Hardness• Distortion• Stresses


Preheating and Postheating (cont'd.)

• Preheating is often required:– On large thick plates

– When plate is very cold

– When temperature is very cold

– When using small diameter electrodes

– On high-carbon or manganese steels

– On complex shapes

– With fast welding speeds


Preheating and Postheating (cont'd.)

• Postheating – Applies heat to metal after welding

– Used to slow the cooling rate

– Reduces hardening

• Interpass temperature– Temperature of metal during welding

– Given as a minimum and maximum


Poor Fitup

• Some welding must be done on joints that are poorly fitted– Requires a good welder

– Skilled welders can watch the molten weld pool and knows how to avoid disaster

• Considerations– Amperage setting may have to be adjusted

– May be necessary to break and restart the arc

– May need to change the electrode angle


Summary

• Grooved welds on one-half inch thick plate – Most common test plates

• Grooved welds – Used by many companies in testing

• Vertical and overhead positions – Most common positions used in testing

• Visually defect-free welds – Assumed to pass destructive testing

– Always make welds as uniform as possible

© 2012 delmar, cengage learning chapter 6 advanced shielded metal arc welding

Documents

cengage learning root

weld pass slide

open root weld

root pass fuses

cengage learning figure

sound weld root

cengage learning cover

cengage learning objectives